1. Field of the Invention
The present invention relates to a radar altimeter having a non-linear signal presence circuit.
2. Prior Art
Radar altimeters, also often called radio altimeters, are used in aircraft to give the pilot an indication of his altitude above ground level (AGL) as opposed to the aircraft's barometric altimeter which is set to display the height above Mean Sea Level (MSL). Generally radar altimeters are used as a landing aid or in ground proximity warning equipment. Therefore, the range of measurement is generally limited to 2500 feet; and many commercial air carriers only display radar altitude to the pilot when the aircraft is below 500 feet AGL.
Frequency modulated-continuous wave (FM/CW) radar altimeters are so named because they transmit a CW signal that is sweeped in frequency as opposed to a pulse signal. The frequency sweep can have any number of shapes but a linear triangle or sawtooth are the most common.
The distance to the ground level, or the AGL altitude, is determined by measuring the frequency difference between the current transmitter frequency and a prior transmitter frequency delayed by the time required for the signal to propagate to the ground and back.
These prior art radar altimeters suffer from inadequate sensitivity to discriminate between relaiable return signals which will produce an accurate altitude reading and return signals which produce erroneous altitude indications.